1. Field of the Invention
The present invention relates generally to a method and apparatus for locating a wireless mobile unit, and more particularly to locating a wireless mobile unit from which a mobile caller is making a distress call.
2. Description of the Related Art
When a caller is dialing an emergency call (E911), it is important for an emergency vehicle responding to the E911 call to accurately locate the caller. For a caller utilizing a wireless unit, such as a mobile caller on a cellular phone, determining an exact location of the caller can be difficult.
Prior art FIG. 1 illustrates a known method for determining a location from which a mobile caller originates a call on a wireless mobile unit 2. Specifically, the E911 call originating from the wireless mobile unit 2 reaches at least one cellular base station, such as base station 4. The signal of the call then typically radiates to a second base station 6 and to a third base station 8. Methods for determining such a location include both time difference of arrival (TDOA) and time of arrival (TOA) methods. These two methods are explained as follows.
Utilizing a known time of arrival (TOA) method as shown in FIG. 1, a first distance xe2x80x9caxe2x80x9d between the wireless mobile unit 2 and first base station 4 is estimated; a second distance xe2x80x9cbxe2x80x9d between the wireless mobile unit 2 and second base station 6 is estimated; and a third distance xe2x80x9ccxe2x80x9d between the wireless mobile unit 2 and third base station 8 is estimated. Based on the time that it takes for the wireless signal to reach each of the base stations, the approximate distances xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, and xe2x80x9ccxe2x80x9d are determined using the known TOA method, as follows.
As shown in FIG. 1, three circles are indicated, one around base station 4, one around base station 6, and one around base station 8. Each of the circles encircle the wireless mobile unit 2. The circles are represented by the following three equations:
((xxe2x88x92x1)2+(yxe2x88x92y1)2)xc2xd=a=C(t1xe2x88x92T)
((xxe2x88x92x2)2+(yxe2x88x92y2)2)xc2xd=b=C(t2xe2x88x92T)
((xxe2x88x92x3)2+(yxe2x88x92y3)2)xc2xd=c=C(t3xe2x88x92T)
In the above-mentioned equations, the coordinates of the wireless mobile unit 2 are represented by (x,y); the coordinates of base station 4 are represented by (x1,y1); the coordinates of base station 6 are represented by (x2,y2); and the coordinates of base station 8 are represented by (x3,y3). Further, t1, t2, and t3 represent half of the round trip delay time of signals traveling from base station 4, base station 6, and base station 8, respectively to wireless mobile unit 2 and back. Finally, T is the processing time of wireless mobile unit 2 and C is the speed of light.
Utilizing the known TOA method of detecting a location of a wireless mobile unit 2, the absolute time of the signal traveling from the wireless mobile unit 2 to the various base stations is measured to find the distances a, b and c and to eventually arrive at an approximate (x,y) location for the wireless mobile unit 2. However, the clock in the wireless mobile unit 2 may not exactly synchronize with that of the various base stations 4, 6, and 8. Accordingly, absolute time is measured by a round trip delay in the time of a signal sent from a particular base station to the wireless mobile unit 2 and back to the base station. However, round trip delay includes processing time in the wireless mobile unit 2 that needs to be estimated. Normally, it can be estimated based on the knowledge of a particular brand of the wireless mobile unit 2.
Another known option for determining a location from which a mobile caller originates a call on wireless mobile unit 2 is to use a time difference of arrival (TDOA) method. TDOA measures the time difference of arrival for signals from the wireless mobile unit 2 to two or more base stations. Hence, the timing factors in the wireless mobile unit 2 are cancelled from the TOA equations expressed above. Assuming that the processing time of the wireless mobile unit is small or known, however, TOA can still be used.
FIG. 2 illustrates an example of a TDOA method involving two hyperbolas with foci at base station 4 and base station 8. The hyperbola xe2x80x9cabxe2x80x9d is constructed using the TDOA between base station 6 and base station 4, in reference to the wireless mobile unit 2. Further, the hyperbola xe2x80x9ccdxe2x80x9d is constructed by using the TDOA between base station 6 and base station 8, in reference to the wireless mobile unit 2. The hyperbolas are determined, with the same coordinates and values previously expressed using TOA, based upon the following equations:
((xxe2x88x92X2)2+(yxe2x88x92y2)2)xc2xdxe2x88x92((xxe2x88x92x1)2+((yxe2x88x92y1)2)xc2xdxe2x88x92C(t2xe2x88x92t1)
((xxe2x88x92X3)2+(yxe2x88x92y3)2)xc2xdxe2x88x92((xxe2x88x92x1)2+((yxe2x88x92y1)2)xc2xdxe2x88x92C(t3xe2x88x92t1)
Accordingly, with these known methods, TOA and TDOA can be used in an ideal situation to determine a location of a wireless mobile unit 2. However, as shown in FIGS. 1 and 2, both the TOA and TDOA methods require the identification of three base stations, 4, 6 and 8 in order to utilize their methods to determine the location of a wireless mobile unit 2. In some instances, however, all three base stations 4, 6 and 8 cannot be determined. An example of this situation is shown in prior art FIG. 3.
The known TOA and TDOA methods of detecting a location of a wireless mobile unit 2, in many instances, only provide an estimated location. Additionally, if all three base stations are not detected, the estimated area cannot even be detected.
For example, as shown in prior art FIGS. 3(a) and 3(b), barriers such as buildings, for example, can block signals from being received by distant base stations. Accordingly, although the cellular call from wireless mobile unit 2 may still be able to go through, only a single base station may detect the call. If only two of the three base stations (6 and 8 as shown in FIG. 3(a) for example) can be detected, only two distances such as xe2x80x9cbxe2x80x2xe2x80x9d and xe2x80x9ccxe2x80x9d can be calculated, resulting in non-unique solutions. If only one or two base stations can be detected, the location of the wireless mobile unit 2 can only be roughly estimated, at best.
As shown in FIG. 3(a) for example, if only two base stations 6 and 8 are identified in a TOA system, then only two circles can be determined and only a parabolic area 10 can be estimated as the approximately location of a wireless mobile unit 2. Similarly, if only base stations 6 and 8 are identified in a TDOA system, as shown in FIG. 3(b), only one parabola xe2x80x9ccdxe2x80x9d can be calculated and the location of the wireless mobile unit 2 can only be roughly estimated at best.
Accordingly, there is a need for a better system and method for determining the location of a wireless unit 2, and especially a mobile caller calling E911.
Utilizing the device and method of the present invention, a position locator is used as a type of floating base station for determining a location of a wireless mobile unit emitting a 911 call. By this arrangement, a distance between the wireless mobile unit and the position locator is estimated, along with the estimated distance between the wireless unit and a single cellular base station, so that the location of the wireless mobile unit emitting the 911 call is accurately determined, even when only one base station receives the call.